I'm asking for advice on how to set up the mechanics of a situation properly. It's not a game run on RPOL and none of the players know me here so no one will see this.

My party will be encountering a group of goblins. The party is 6 people, balanced as any normal party is with magic-users and fighters. The encounter will take place in a sparsely populated, hilly forest. It's crucial to the game plot that the players fight the goblins for at least one round and then the goblins have to escape. They need to escape because the party will need to face them a second time in order for them to discover a part of the plot.

Does anyone have any advice on how I can set up the encounter so that the goblins escape with absolutely no chance that the party can hunt them down until the second encounter? I'd prefer to do it in a non Deus Ex Machina way to avoid party members grumbling.

You can't guarantee that all the goblins will escape without the PCs killing them.  Be prepared to lose at least one or two.  What you can do is stack the deck in the monster's favor.

The goblins choose the ambush site, and goblins are nasty under that kind of condition.  They get cover, the PCs don't.  The ground between the PCs and the goblins is either difficult terrain (PHB190), trapped (DMG120 - Simple Pit is a good choice), or worse. The ground the goblins flee over is trapped.  The goblins flee through a cave... which is also trapped.

You get the idea.  Also, don't forget that goblins are fast.  They get a bonus action to disengage or hide every round, which really boosts speed when you're running away.

Do all of the goblins have to escape? If not, then your answer is numbers. Take however many goblins need to escape, and add four or five. They don't even all have to be actively fighting. Some might be hanging back as reinforcements or something. And then, however many rounds in, some of them run off. There's no way they can chase/catch them all. If there are some 'Named' goblins that you need later, put the redshirt gobbos in the fight, and the Named gobbos on the sidelines, directing.

If all the goblins have to escape, then what you want to do is place a visible, obvious terrain element on the battlefield that will allow the goblins to escape.

Like, maybe, a lever that will collapse a dam and let loose a river onto the PC's. Some kind of trap involving a pile of cut trees or logs or fallen rock or something similar.

Or even a cave with a collapsible entrance. They run in, rocks fall, PC's can't enter (or maybe they can, but by the time they do the little guys are long gone).

Or you could add something to the environment that would obfuscate their escape. Like, if the fight happened near a massive wasp nest. Or ant hill. Or maybe they have a lot of rabid, hungry dogs in cages and they open the cages and off they go.

Well, I'd start off my recommending against setting things in stone so firmly. Because players will always find ways to ruin inflexible ideas and plans.

Do all the goblins have to escape? If not, you can have some of the goblins up in the trees as sharpshooters: after the first round, the goblins can forest-parkour through the canopies of the trees to escape. It will be difficult at low level for the parties to pursue or take them out if the goblins have the advantage of running away through the trees.

If this is set up as an ambush, the goblins could pop up from tunnels (Viet Cong style), attack the party for a round, then vanish back down into the labyrinthine tunnels below.

If the goblins have a shaman-type with them, Fog Cloud can be used to obscure the area and buy the goblins an opportunity to escape.

I'm not too good on 5e from a GM's perspective; but I'll just offer one piece of advice; backing up what others have said:

Do not have something that -has- to happen. Because creative players will stuff up the best laid plans of mice and GMs...

Mechanically I can't help you. But GM-wise I'll also say never rely on any particular outcome when players are involved.

In the event all goblins die, one of them can have a map or note that will push the story forward, by indicating where the party should go and possibly setting up a meeting with a second group of goblins.

Or you could posit Goblins who had been hiding, but did not get involved in the battle. Up in a tree or something.

In reply to Piestar (msg # 7):

Be -very- careful with that tactic. If the players figure out you did that, they'll feel cheated, and become disillusioned. If you do it; I'd recommend making sure you do hide checks and the like - give them an insane hide modifier, sure. But make sure there is a chance, however slim, that they can be seen. Then they can run like hell but still.

Changing things midway just because the players do something you didn't expect, in this case kill all the enemy, is the first step to players starting to lose interest if they find out, because they'll feel "Well it doesn't matter; he'll just somehow make sure we can't have total victory". Plans being derailed is preferable to that.

Yeah, make it hard.  Stack the deck in the favor of the goblins... but if the PCs pull off a miracle, don't take that away from them (last night, the wizard remembered he had a gem containing an elemental that he got a dozen sessions before and had never used - changed the tide of several battles).

You can always find another way to give them the clue.

I would ask why the goblins have to escape in the first place - as others said, if you need to give the players the clue, why can't they get it the first time around?

Also, remember that the goal of a roleplaying game is to let the player make meaningful decision - if you decide the outcome of their decision before they made it, they lose meaning. What if the players used glamour to pass themselves off as goblins and infiltrate the enemy forces? In such a scenario, you'd be unable to have the fight, and the players also would be able to follow the goblins to their lairs, completely derailing your preconceived outcome. If you think that couldn't happen, know I have had it sprung on me several times, so it's far from farfetched.

And if you fudge the goblins' saves to keep your predetermined outcome, what's the point of the players having glamour spells in the first place? Their decision to use them becomes meaningless, since you'll never let them succeed.

As such, I do agree that you should create a resonable scenario to favour the goblins' survivals (snipers and traps and numbers, as said by others, are good way to do it), but also to make plans so that, if things do not go the way you hoped for, you have other recourses - like the goblins having some clues on heir persons, or their disappearance triggering action from others NPCs, and more such things.

Do not be afraid of the players doing the unexpected and twarting your plans; the key to GMing is rolling with the punches, so do not predetermine the encounter's outcome - instead, plan the world around the encounter so that, no matter what the outcome is, you'll still be able to deliver an interesting story to the players - without stripping their choices of meaning.

You might even find that the story the players derail you into is more interesting then the one you hoped for, and that's much better than predetermining the outcomes of their actions to have things your way! ^_^

Consider giving the goblins a small stable of mounts (worgs are always good, but horses, ponies, mules, or more exotic beasts can work well, too) they can ride to outpace PCs. Vietcong-style tunnels is one way to provide an escape route the PCs can't easily follow, but you could also simply add a cliff that runs adjacent to the ambush site that the PCs will have difficulties scaling before the goblins can escape to the scene.

The goblins probably know the area well enough not to need a map*, but unless the PCs explicitly execute the goblins or deal enough damage to kill them all outright, you can always say that one of the wounded goblins are still alive to provide that information. One of the retreating goblins might also step in a trap, twist his ankle or break a leg while retreating, or simply be the retreating goblin who isn't fast enough to escape the PCs**.

*During a 3.5 game I ran back in the day, the PCs assumed that the map they found on one of the kobolds was a trap (although to be fair this pack of kobolds were fond of traps to the point of ridiculousness so I couldn't blame them for being cautious), and during another game with a different group the PCs, the PCs didn't loot bodies as a matter of principle so they never found the map.

**Having one retreating NPC being slower than the rest will force the PCs to stop or slow down to deal with the slow NPC, which gives the faster group yet more time to escape. Or if they focus on the faster group, you can have the slow one get away.

You can also try to make the ambush encounter lead more directly to the second encounter. It might help to make the first ambush a little easier so there's less need for the PCs to rest inbetween the two encounters.

Final suggestion: Why keep it simple and realistic when you can make it cool and fantastical? Give the goblins a rudimentary system of platforms, primitive gondola lifts, zip-lines, and so forth that they use to navigate the forests quickly, circumventing the difficult terrain of the forest floor entirely. The players can follow the system easily when they want to pursue the goblins (mitigating the need to shoehorn in maps, interrogations, or what have you), or even use it to immediately pursue the goblins without pausing for a rest (naturally it'll take a round at the least to get everyone up). Put up some adjacent zip-lines and have an additional encounter in the sky where goblins and PCs fight dozens or hundreds of meters above the ground -- goblin reinforcements arriving by hang-glider as necessary.

The real question to ask here is whether your PCs tend to follow the "go here" clues you leave them (map, interrogation) or if they're more cautious and need to be lead more subtly (link encounters, tracks, platforms and ziplines acting as less obvious clues while still providing the metaphorical rails of the adventure).

Size here would be useful unless all your PCs are halflings and gnomes.  rope bridges that aren't strong enough to hold a medium PC without a save or not hold two at all.  As well as wood supported earthen tunnels only large enough for small creatures to crawl.

But in each case make sure that they have a chance to learn some weakness in these goblin techniques such as air vents, or extra flammability of the ropes that will make the next attack against it easier.

Easy outside threat. Have something or someone else show up and intrupt. You can usually do a cenamitic scene shift and no one gets upset. Even fleeing can be done that way if the fight is good enough to keep the players from wanting more so soon.

An example could be...

A T-rex tramps out the forest disturbed by your noise. The goblins waste no time in turning to run and before you can reliaze there gone 9 tons of teeth and claws are coming your way.

Having them escape through a tunnel that is only wide enough for at most one small creature at a time, potentially starting as a hidden hollowed out tree for instance.  If the PCs include a small folk... who could potentially follow... you could place a trap in the tunnel where the last or second to last goblin out lets loose a poisonous snake.  If the last goblin dies in the tunnel there isn't enough room to get around him to continue.  Where the the tunnel comes out may be near a set of obsolesces that make following tracks impractical.  Could be a combination of running water, a set of vines that could have been used to transfer over to a different area, or even have been magically enchanted to keep track from appearing for anyone in the area.

I agree that have a plot which you can't imagine anyway of dealing with an unexpected turn is dangerous, as players will often do the unexpected.  I don't know how many times we went through a module and found out afterwards that we managed to go through significant portions of it backwards, which created a interesting dynamic of some things being easier, but some being harder, because of items we were presumed to have picked up already were a few rooms ahead of us.